"In Lewis and Clark, we can see American idealism and the breathtaking natural beauty of the continent, as well as the complexities and tragedies of what came to be known as America's 'manifest destiny,' " said HBO programming president Michael Lombardo. "We are tremendously excited to bring together this incredible group of talents to tell a seminal American story."

Based on Stephen E. Ambrose's book about the famous duo, Undaunted Courage, the project has been gestating at the pay cable network for quite some time. Affleck came onboard last summer, but Schoenaerts (star of 2012 Cannes breakout Rust and Bone) just joined the project.

The drama hails from HBO and National Geographic Entertainment and is produced by Playtone, Plan B and Class 5. Its bold-faced list of executive producers includes Tom Hanks, Gary Goetzman, Edward Norton, Brad Pitt, Dede Gardner, Tim Kelly and Adam Leipzig.

At six hours, it will also be the most sizable miniseries HBO has put out since 2010's The Pacific.